1. Field of the Invention
This invention relates to tools that may be used in wells. More particularly, method and apparatus are provided for decreasing drag force in the trigger mechanism (a collet, mandrel, sleeve combination) of a jar used for releasing stuck objects in a well.
2. Description of Related Art
Jars are tools that are widely applied in wells for releasing an object stuck in the well. Mechanical jars store potential energy in a support above the jar and use a release mechanism to apply the energy as an impact force on the housing of the jar. One form of release mechanism is a collet that is adapted to release a mandrel when a sleeve disengages, allowing a hammer on the mandrel to impact an anvil on the housing. Examples of jars employing a collet, collar and mandrel as a release mechanism are described in U.S. Pat. Nos. 6,290,004; 6,481,495; 6,988,551; and U.S. Patent Application No. 2006/0169456, all having common inventorship with the present disclosure.
Frictional forces decrease the efficiency of mechanical devices such as collet-and-mandrel combinations. The frictional force created by relative motion of the collet and mandrel can have a great effect on the operation of such apparatus. In a jar, frictional force can significantly affect the mechanical impulse applied to the equipment that is stuck. Frictional force can also retard the actions necessary to reset the device. Therefore, there is need for means to decrease frictional drag in devices employing a collet mechanism to restrain a mandrel and release it at a selected position.
Methods used to decrease frictional force include use of lubricating coatings on surfaces and immersing surfaces in lubricating fluids. A further way to decrease frictional drag between surfaces is to decrease the contact force between the surfaces. There is a need for method and apparatus for decreasing the contact force between a collet and a mandrel in apparatus, such as a jar, in which the collet is used to release the mandrel at a selected position.